1. Installation

unpack the archive

The .zip archive from GitHub is structured (folder hierarchy) as you find it in their local Gimp install directory.
Copy all files/ folders in this directory "C:\Program Files\GIMP 2\share\gimp\2.0" or use Gimp User Folder "C:\Users\Your User Name\.gimp-2.8\"

Now start Gimp

template

for a quick start, you can use the template "pixelated.001.xcf"

2. Prepare the picture

Open an image of your choice (RGB, RGBA)

good results I have conceded with image sizes of 2500px and higher (picture 1) create a new layer above the "background" layer called "area-brush" (picture 2). With the "area-brush" layer selected, brush onto the areas of your photo that you want to apply the effect to.